Characteristics of pronuclear migration in Beroe ovata

In the large eggs (approximately 1 mm) of the ctenophore Beroe ovata, female pronuclei migrate long distances to join stationary male pronuclei in the peripheral cytoplasm that surrounds the yolky interior. We have investigated the mechanism of nuclear migration using time lapse video recording, automated image analysis, visualization of microtubules by immunofluorescence and rhodamine-tubulin injection, and electron microscopy. Female pronuclei migrated at average speeds of 0.2 microns/sec, and were found to show periodic oscillations in velocity. Alternating phases of acceleration and deceleration occurred with an average periodicity of 235 seconds covering distances of 47 microns (about 3 times the nuclear diameter). Migration velocities and velocity oscillations were similar in fertilized and unfertilized eggs; however, changes in migration direction were much more frequent in unfertilized eggs. Characteristic deformations of the pronuclear membrane and occasional rotation of the nuclear contents were observed during migration. Inhibitor studies indicated that microtubules are required for nuclear migration. In fertilized eggs the top of the nucleus was found to move through the dense layer of aligned sperm aster microtubules. The frequent changes in direction of pronuclear migration in unfertilized eggs reflect the random organization of the microtubule layer in the absence of sperm derived centrosomes. Densely packed endoplasmic reticulum was found intermeshed with sperm aster microtubules and connected extensively with the nuclear membrane during migration. Most nuclear pores were grouped in an infolding of the nuclear membrane. We suggest that in fertilized eggs the female pronucleus is transported to the minus ends of sperm aster microtubules using motor molecules attached either to the outer nuclear membrane and/or to the network of connecting ER.